1. Field of Invention
This invention relates to a system and method for manufacturing fiber reinforced thermoplastics into structures having relatively large dimensions. More particularly, the present invention relates to a system and method for manufacturing high modulus and high strength fiber reinforced thermoplastic composites whose product configuration is characterized by a non-uniform cross section or non-linear elongate axis.
2. Prior Art
Although fiber reinforced thermoplastics (FRTP) have only been in commercial production for several years, such composites are receiving increased attention and popularity because of their versatility. For example, composites made of thermosetting resins are limited because of curing procedures which involve cross linking of polymers to a final, irreversible molecular structure. Thermoplastic resins, on the other hand, are stabilized and may be reversibly modified to change configuration. Despite the versatility of the FRTP material, however, fabrication procedures have been limited and somewhat inhibiting to applications of such composite materials for commercial products.
Generally, five methods are recognized as representative of fabrication techniques for FRTP composites. The first of these methods is referred to as matched die forming. In this process, the thermoplastic composite laminate is heated outside a mold and then transferred to the mold area and pressed between two non-heated, matched, rigid molds which are mounted on platens of a press. When this procedures involves replacement of one side of the mold with an elastomer, the process is referred to as hydroforming. Another category within this group is commonly referred to as compression molding, but is primarily used for thermosetting resins, where the material is placed between heated, matched dies. Curing occurs within forming cavities of the molds. Such matched die forming procedures require a press and an oven that is at least the size of the part to be produced. Where parts are large, such as a wing section for aircraft, the size of such a press and oven may stretch beyond economic feasibility.
Another classification of techniques for forming FRTP composites is identified as autoclaving. In this process, the laminate layers are laid in a mold which is sealed with a vacuum bagging assembly. The mold is then placed in the autoclave and slowly heated. Pressure is applied to reduce the void content in the material and increase its mechanical properties. After a time at the softening point temperature, the mold is slowly cooled, allowing for release of the molded product. The disadvantages of autoclaving are similar to those of matched die forming, in that the size of the part is restricted by the size of the autoclave. Forming large composite structures by these methods is often cost prohibitive.
A similar method of FRTP fabrication is referred to as continuous heating, roll forming. This method is similar to roll forming of metals in that a continuous length of thermoplastic composite is passed through a heating section and into a series of forming rollers. This process produces long parts with a constant or uniform cross sectional area. It is similar to pultrusion technology applied to thermosetting resin composites. Although this process can create large composite parts, such parts are limited to cross sections of uniform shapes such as hat sections, C-channels, rods, etc. Techniques for varying cross sectional configuration along the length of a formed or forming structure have been unknown and perceived as infeasible within a competitive manufacturing operation.
Filament and tape winding is a fourth method sometimes used with thermoplastic compositions, as well as with thermosetting resins. In this procedure, a tow or tape of fibers is impregnated with thermoplastic resins. This impregnated tow is heated and applied to a rotating mandrel configured in accordance with desired product shape. Consolidation of the material is provided by a pressure roller. Obviously, the very manner of formation limits such processes to surfaces of revolution and are not well suited for formation of laminate parts on mass production scale.
Finally, a fifth category of fabrication involves diaphragm forming. In this process, a thermoplastic layup packet is placed between two superplastic alloy sheets. The laminate and the cover sheets are placed in an oven. The materials are heated and then pressure formed into a mold. Here again, the fabricated parts are limited in size to the size of the heating and pressure vessel applied. Furthermore, the use of superplastic alloy sheets are costly and not well suited for mass production.
Of these five methods, the dominant fabrication techniques involve the four types which produce component parts by heating and compression of the total part within a single mold. Capital costs in preparing such mold structure are often prohibitively high. For example, larger molds may cost tens or hundreds of thousands of dollars just to have the mold cut to proper configuration. Other capital costs involved in operating machinery to make single components must be added on top of these high mold costs. Such cost increases tend to be geometric and can greatly increase the cost of production where no other options are available.
Such options are limited because continuous production line systems such as pultrusion and roll forming are only useful for parts having uniform cross sections and a linear axis. This is not to say that some pultruded FRTP composite structures may not be heated and bent into other shapes; however, fabrications of non-linear parts and parts having non-uniform cross sections have not been successfully introduced to the plastic industry on a large commercial scale. Furthermore, commercial applications of roll forming, pultrusion and extrusion have generally been limited to die cavities which are both linear and of uniform cross section such that the interception of two parallel planes through the formed composite or mold cavity are geometrically congruous.